Quick starting magnetron with shielded cathode

ABSTRACT

A quick starting magnetron having two cathodes, one of low heat inertia and thermally emissive for starting purposes and the second of high heat inertia positioned between the magnetron anode and the first cathode. The second cathode is apertured to allow electrons to pass through from the first cathode and is arranged to partially shield the first cathode from electron bombardment during normal running of the magnetron.

United States Patent [1 1 Esterson et a1.

1 1 QUICK STARTING MAGNETRON WITH SHIELDED CATHODE [75] Inventors:Maurice Esterson; Francis James Weaver, both of Chelmsford, England [73]Assignee: English Electric Valve Company Limited, Chelmsford, England[22] Filed: Dec. 13, 1973 [21] Appl. No.: 424,358

[451 Aug. 12, 1975 2,818,528 12/1957 Feinstein 313/338 X 2,822,4992/1958 Lynch 313/338 X 3,297,901 l/l967 MacDonald et a1. 313/337 X3,403,281 9/1968 Weaver 313/337 3,505,557 4/1970 Schmidt n 313/338 XPrimary ExaminerSaxfield Chatmon, Jr. Attorney, Agent, or FirmBaldwin,Wight & Brown 1 1 ABSIRACT A quick starting magnetron having twocathodes, one of low heat inertia and thermally emissive for startingpurposes and the second of high heat inertia positioned between themagnetron anode and the first cathode. The second cathode is aperturedto allow electrons to pass through from the first cathode and isarranged to partially shield the first cathode from electron bombardmentduring normal running of the magnetron.

10 Claims, 1 Drawing Figure Mao/p 1 QUICK STARTING MAGNETRON WITHSHIELDED CATHODE This invention relates to magnetrons and has for itsobject to provide improved quick starting magnetrons of high powercapability.

It is well known to obtain quick starting qualities in a magnetron bymaking the cathode structure of such design that it will heat up veryrapidly to emission temperature from cold. However, known cathodestructures of such design have the defect that because, after startingand during normal running of a magnetron, the cathode is subjected toconsiderable electron bombardment, the cathode temperature tends to risesubstantially and this sets an undesirable limit to the power which canbe safely handled. Indeed it is not uncommon to make magnetrons withheated cathodes the heaters of which are switched on only duringstarting, electron bombardment during running serving alone to maintainthe cathode at normal operating temperature. To be quick starting amagnetron must have a thermionic cathode of low heat inertia and such acathode is necessarily of poor heat dissipation properties usuallythrough being of poor heat conductivity and accordingly, if, afterstarting, the cathode is subjected to too much additional heating byelectron bombardment, the heat is unable to get away and the temperaturerises to an unacceptably high level. This limits the power capability ofthe magnetron very severely and known quick starting magnetrons arelimited in this way to power levels at which electron bombardmentheating of the quick starting cathode will not cause its temperature torise to the point at which it may be damaged or destroyed. The presentinvention seeks to lift this limitation and to provide improved quickstarting magnetrons of higher power capability than comparable quickstarting magnetrons known to the applicants.

According to this invention a quick starting magnetron has an anode, afirst thermally emissive cathode structure of low heat inertia; and asecond cathode structure of relatively high heat inertia positionedbetween said anode and said first cathode structure and apertured so asto allow electrons to pass therethrough from said first cathodestructure into the space between the anode and the second cathodestructure; said second cathode structure being positioned anddimensioned so as to at least partially screen the first cathodestructure from electron bombardment during normal running of themagnetron.

Preferably the second cathode structure is given a sufficiently highsecondary emission coefficient to ensure that practically all or themajor part of the electron current required for magnetron operation issupplied by secondary electron emission.

The second cathode structure may be designed to be heated only byelectron bombardment but,-if desired, it may also be arranged to beheated electrically.

In a preferred embodiment the first cathode structure comprises a thinwalled inner metal tube having a ring coating of thermionically emissivematerial thereon and said second cathode structure comprises, co-axialwith said thin walled tube, a thick walled tubular structure aperturedaxially adjacent said ring coating. Electrical heating of the thinwalled tube may be effected in any of a variety of known ways butpreferably it is accomplished directly by passing heating currentthrough it. In a preferred arrangement of this type a central axial rodis provided inside and spaced from the thin walled metal tube; said rodis fixed at its outer end to an outer end hat" of disc form which alsolocates the outer end of the thin walled tube and the outer end of thethick walled tubular structure; and an inner annular end hat" surroundssaid rod and locates the inner end of the thick walled tubularstructure, means being provided for feeding heating current through acircuit including the axial rod, the outer end hat and the thin walledtube.

The last mentioned means preferably includes said support rod and anadditional support which is hollow, e.g., cylindrical, surrounds saidrod and supports both the inner end hat and the inner end of the thinwalled tube. In such an arrangement the thick walled tubular structuremust not short circuit the thin walled tube and therefore there must bean insulating gap in the electrical path from one end hat to the otherthrough the tubular structure. This could be done by insulating one endof the tubular structure from the associated end hat but preferably thetubular structure is formed by two cylindrical sections separated onefrom the other, with the gap therebetween forming the aperture forelectrons from the thin walled first cathode structure.

In this arrangement the support rod and additional support provide goodthermal conduction paths from the thick walled structure to the exteriorof the magnetron.

The invention is illustrated, by way of example, in the accompanyingdrawing which illustrates schematically, and so far as is necessary toan understanding thereof, a prefered embodiment.

The magnetron represented in the drawing is, except for the cathodearrangement, a multi-cavity magnetron of well known form. Accordinglyonly the said arrangement is shown in any detail, the rest of themagnetron being represented merely by the inner ends, of the radialvanes which are fixed to the inside of the anode cylinder to provide aplurality of resonant cavities in accordance with well known magnetronpractice.

Referring to the drawing, V are the inner ends of two of the vanes whichproject radially inwards from the cylindrical anode of the anode systemof a multi-cavity magnetron. Axially situated within the anode system isa cathode arrangement which comprises a quick staring first cathodestructure and a second main cathode structure which acts also as anelectron shield to prevent overheating of the quick starting cathode andits support by bombardment by returning electrons when the magnetron isin normal running operation.

The quick starting cathode structure is constituted by a ring 1 ofsuitable thermionic emissive material on the outer surface of athin-walled metal tube 2 at about the middle of the length thereof.Because of the thinness of the wall of the tube 2, the quick startingcathode is of low thermal inertia. For the same reason, however, it isunable to conduct heat away quickly for dissipation.

Co-axially surrounding the quick starting cathode is a combined maincathode structure and electron shield. This consists of two similarthick walled metal tube portions 3, 4 on the outer surfaces of which aredeposits 5, 6 of material capable of secondary electron emission. ifdesired material capable of primary electron emission may also bedeposited over all or part of the surface. There is, between theadjacent ends of the portions 3, 4, a gap or aperture 7 which isopposite the emissive ring 1 and through which electrons from said ringcan pass into the working electron space inside the anode system.

8 and 9 are metal so-called end hats" which locate the ends of thecathode structures and an axial metal rod 10 extends up to and isconnected to the upper end hat 8. 11 represents a main electricallyconductive support for the cathode structure as a whole and the lowerend hat 9 sits on this support. Heating of the quick starting cathode iseffected electrically by passing heating current through a heatingcurrent circuit consisting of the rod 10, the upper end hat 8 the thinwalled tube 1 and the support 11.

An arrangement as illustrated enables a quick starting magnetron ofsubstantially high power capability to be obtained because, owing to theelectron shielding of the thin walled cathode structure provided by thetube portions 3 and 4, over-heating of the thin walled tube 2 and theemissive cathode material 1 carried thereby will not now occur.Additionally the rod 10 and support 11 provide a good thermal conductionpath for heat form the thick walled structure 3, 4 to the exterior ofthe magnetron so as to permit operation of the magnetron at levels ofpower which cause substantial back heating of the structure 3, 4 byelectron bombardment.

The design may be such that the heating current is kept switched onafter starting or it may be such that the heating current is switched ononly for starting and then switched off. As illustrated the main cathode(3, 5 4, 6) is not provided with any heating means (other, of course,than by electron bombardment by returning electrons) but, if desired,electrical heating for this cathode may be arranged for in anyconvenient manner known per se and the design may be such that thiselectrical heating is switched on only during starting or,alternatively, is kept on after starting.

We claim:

I. A quick starting magnetron including an anode; a first thermallyemissive cathode structure of low heat inertia; and a second cathodestructure of relatively high heat inertia positioned between said anodeand said first cathode structure; a continuous circumferential apertureprovided in said second cathode so that said second cathode is formed intwo axially separated portions, the aperture permitting electrons topass therethrough from said first cathode structure into the spacebetween the anode and the second cathode structure; said second cathodestructure being positioned and dimensioned so as to at least partiallyscreen the first cathode structure from electron bombardment duringnormal running of the magnetron.

2. A quick starting magnetron according to claim 1 wherein the secondcathode structure is given a sufficiently high secondary emissioncoefficient to ensure that the major part of the electron currentrequired for magnetron operation is supplied by secondary electronemission.

3. A quick starting magnetron according to claim 1 wherein the secondcathode structure is arranged to be heated only by electron bombardment.

4. A quick starting magnetron according to claim 1 wherein the firstcathode structure is arranged to be heated electrically.

5. A quick starting magnetron according to claim 1 wherein the firstcathode structure comprises a thin walled inner metal tube having a ringcoating of thermionically emissive material thereon and said secondcathode structure comprises, co-axial with said thin walled tube, athick walled tubular structure apertured axially adjacent said ringcoating.

6. A quick starting magnetron according to claim 5 wherein the thinwalled tube is electrically heated by passing heating current throughit.

7. A quick starting magnetron according to claim 6 wherein a centralaxial rod is provided inside and spaced from the thin walled metal tube;said rod is fixed at its outer end to an outer end hat of disc formwhich also locates the outer end of the thin walled tube and the outerend of the thick walled tubular structure; and an inner annular end hatsurrounds said rod and locates the inner end of the thick walled tubularstructure, means being provided for feeding heating current through acircuit including the axial rod, the outer and hat and the thin walledtube.

8. A quick quick starting magnetron according to claim 7 wherein themeans for feeding heating current includes said support rod and anadditional support which is hollow cylindrical, surrounds said rod andsupports both the inner end hat and the inner end of the thin walledtube and wherein there is an insulating gap in the electrical path fromone end hat to the other through the tubular structure.

9. A quick starting magnetron as claimed in claim 1 wherein the apertureis provided mid-way along the axial length of said second cathodestructure.

10. in a quick starting magnetron having an anode space defining aplurality of magnetron cavities, the combination of:

a first cathode structure disposed within said anode space andcomprising a thin wall tube having a ring of thermally emissive materialon its outer surface;

conductor means engaging opposite ends of said tube for supporting saidtube and passing heating current therethrough whereby to provide quickstarting of the magnetron; and

a second cathode structure surrounding and spaced from said tube andhaving aperture means aligned with said ring of thermally emissivematerial to allow electrons to pass into said anode space, said secondcathode structure including thick wall tube portions on either side ofsaid aperture means shielding said thin wall tube from bombardment fromreturning electrons, said conductor means engaging said tube portionsfor conducting heat therefrom to the exterior of the magnetron, and saidtube portions being provided with exterior coatings of secondaryemission material.

1. A quick starting magnetron including an anode; a first thermallyemissive cathode structure of low heat inertia; and a second cathodestructure of relatively high heat inertia positioned between said anodeand said first cathode structure; a continuous circumferential apertureprovided in said second cathode so that said second cathode is formed intwo axially separated portions, the aperture permitting electrons topass therethrough from said first cathode structure into the spacebetween the anode and the second cathode structure; said second cathodestructure being positioned and dimensioned so as to at least partiallyscreen the first cathode structure from electron bombardment duringnormal running of the magnetron.
 2. A quick starting magnetron accordingto claim 1 wherein the second cathode structure is given a sufficientlyhigh secondary emission coefficient to ensure that the major part of theelectron current required for magnetron operation is supplied bysecondary electron emission.
 3. A quick starting magnetron according toclaim 1 wherein the second cathode structure is arranged to be heatedonly by electron bombardment.
 4. A quick starting magnetron according toclaim 1 wherein the first cathode structure is arranged to be heatedelectrically.
 5. A quick starting magnetron according to claim 1 whereinthe first cathode structure comprises a thin walled inner metal tubehaving a ring coating of thermionically emissive material thereon andsaid second cathode structure comprises, co-axial with said thin walledtube, a thick walled tubular structure apertured axially adjacent saidring coating.
 6. A quick starting magnetron according to claim 5 whereinthe thin walled tube is electrically heated by passing heating currentthrough it.
 7. A quick starting magnetron according to claim 6 wherein acentral axial rod is provided inside and spaced from the thin walledmetal tube; said rod is fixed at its outer end to an outer ''''endhat'''' of disc form which also locates the outer end of the thin walledtube and The outer end of the thick walled tubular structure; and aninner annular ''''end hat'''' surrounds said rod and locates the innerend of the thick walled tubular structure, means being provided forfeeding heating current through a circuit including the axial rod, theouter and hat and the thin walled tube.
 8. A quick quick startingmagnetron according to claim 7 wherein the means for feeding heatingcurrent includes said support rod and an additional support which ishollow cylindrical, surrounds said rod and supports both the inner endhat and the inner end of the thin walled tube and wherein there is aninsulating gap in the electrical path from one end hat to the otherthrough the tubular structure.
 9. A quick starting magnetron as claimedin claim 1 wherein the aperture is provided mid-way along the axiallength of said second cathode structure.
 10. In a quick startingmagnetron having an anode space defining a plurality of magnetroncavities, the combination of: a first cathode structure disposed withinsaid anode space and comprising a thin wall tube having a ring ofthermally emissive material on its outer surface; conductor meansengaging opposite ends of said tube for supporting said tube and passingheating current therethrough whereby to provide quick starting of themagnetron; and a second cathode structure surrounding and spaced fromsaid tube and having aperture means aligned with said ring of thermallyemissive material to allow electrons to pass into said anode space, saidsecond cathode structure including thick wall tube portions on eitherside of said aperture means shielding said thin wall tube frombombardment from returning electrons, said conductor means engaging saidtube portions for conducting heat therefrom to the exterior of themagnetron, and said tube portions being provided with exterior coatingsof secondary emission material.